The Marble Arch Caves are formed by three rivers that drain off the northern slopes of Cuilcagh mountain, meet underground and emerge as the Cladagh River at the largest karst spring in Ireland. With 11.5 kilometres of known passages, it is the longest cave system in Northern Ireland, and the show cave, opened in 1985, is the most active river show cave in Ireland and the UK.
Guided tours begin with a short boat ride on the underground river and continue on foot along paths through chambers where discreet lighting picks out the cave formations. Book ahead in summer. Heavy rain can raise the water and stop tours, so check before you set out. Above ground, the walk down the wooded Cladagh Glen to the natural limestone arch that gives the caves their name is worth doing in its own right.